Some optical communication devices that receive WDM (Wavelength Division Multiplexing) signal light perform ALC (Automatic Level Control) so that the level of received signal light is constant at a predetermined value. In ALC, the level of signal light is controlled to be constant at a predetermined value by monitoring the output power of an amplifier and controlling the output power of the amplifier to be constant.
Moreover, there is a case where signal light may include ASE (Amplified Spontaneous Emission) light when amplified by an amplifier. Therefore, if the power of the ASE light included in the output power of the amplifier becomes relatively large compared to the power of the signal light, the level of the signal light is controlled to be constant, at a value smaller than a value to be controlled constant by ALC. Consequently, there is known ASE correction to correct the level drop of the signal light due to the ASE light.
The ALC and ASE correction need information of number of wavelengths of signal light. It is possible for an optical communication device that performs ALC and ASE correction to know the number of wavelengths of signal light by, for example, an OSC (Optical Supervisory Channel).
Note that, conventionally, a wavelength conversion device is provided, which extracts one or two or more beams of signal light (signal light to be subjected to wavelength conversion) to be subjected to wavelength conversion from a plurality of beams of signal light transmitted by WDM with an optical part having the filtering function to give light to be wavelength-converted and performs wavelength conversion on the light (for example, see Japanese Laid-Open Patent No. 2001-249368).
However, there is such a problem in the conventional optical transmission system that the optical communication device on the reception side, to perform ALC or ASE correction, needs a device to transmit information of number of wavelengths to the optical communication device on the transmission side, and therefore, the cost of the optical transmission system is raised.
For example, the optical communication device on the transmission side needs a device to transmit an OSC including information of number of wavelengths, and therefore, the cost of the optical transmission system is raised.